1. Field of the Invention
The present invention relates generally to the field of direct oxidation fuel cells for producing electrical energy by electrochemical oxidation/reduction of an organic fuel, and in particular to a direct oxidation fuel cell with integrated water management.
2. The Prior Art
Fuel cell technologies present opportunities for the commercial development of long-lasting power sources for portable power and electronics applications. With the trend toward greater portability of a wide array of consumer electronics, some fuel cell technologies offer promising alternative power sources to meet the increased demand for portable power. Fuel cells can potentially replace or favorably compete with the various types of high density batteries presently used in consumer electronics, such as nickel metal-hydride and lithium ion battery systems, as well as relatively inexpensive alkaline batteries. These types of batteries are less than satisfactory power sources for such consumer electronics as laptop computers and cellular phones either due to their low power density, short cycle life, rechargability or cost. In addition, all these types of batteries present environmental safety concerns and costs for proper disposal.
Fuel cell systems are electricity-generating devices that convert chemical energy into useable electrical energy via a simple electrochemical reaction involving a fuel reactant, such as natural gas, methanol, ethanol, or hydrogen, and an oxidizing agent, typically ambient air or oxygen. Fuel cell systems may be divided into xe2x80x9creformer-basedxe2x80x9d systems, i.e., those in which the fuel is processed in some fashion before it is introduced into the cell, or xe2x80x9cdirect oxidationxe2x80x9d systems, i.e., those in which the fuel is fed directly into the cell without internal processing. Most currently available stationary fuel cells are reformer-based fuel cells. However, fuel processing requirements for such cells limits the applicability of those cells to relatively large systems.
Referring to FIG. 1, a conventional direct oxidation fuel cell 1, wherein the fuel reactant 3 is fed directly into the fuel cell 1 without internal modification or oxidation, is typically constructed of an anode diffusion layer 5, a cathode diffusion layer 7, and an electrolyte 9, such as a protonically conductive electronically non-conductive membrane electrolyte (xe2x80x9cPCMxe2x80x9d), that is disposed between the anode and cathode diffusion layers. Fuel reactant is introduced into the fuel cell anode and is presented to a catalytic layer 11 intimately in contact with the anode face of the PCM. The anode catalyst layer separates hydrogen from the fuel reactant into protons and electrons as a result of oxidation. Upon the completion of a circuit which electrically connects the anode and cathode of the fuel cell, protons generated by the anodic catalytic reaction pass through the membrane electrolyte to the cathode of fuel cell. Electrons generated by anodic oxidation of fuel molecules cannot pass through the membrane electrolyte, and seek a path through the load which is being powered. The electrons flow away from the anode catalyst, through the anode diffusion layer, and are collected by a current collector 10, pass through a load (not shown), through a current collector 12, through the cathode diffusion layer and to the cathode catalyst layer 13 where the electrons combine with protons and oxygen to form water.
As long as constant supplies of fuel reactant and an oxidizing agent are available to the fuel cell, it can generate electrical energy continuously and maintain a desired power output. Hence, fuel cells can potentially run laptop computers and mobile phones for several days rather than several hours, while reducing or eliminating the hazards and disposal costs associated with high density and alkaline batteries. A further benefit is that a fuel cell runs cleanly producing water and carbon dioxide as by-products of the oxidation/reduction of the fuel reactant. The challenge is to develop fuel cell technology and to engineer direct fuel cell systems to meet the form and operation requirements of small-scale or xe2x80x9cmicroxe2x80x9d fuel cells for portable electronics applications.
Direct methanol fuel cell (xe2x80x9cDMFCxe2x80x9d) systems are often multi-cell xe2x80x9cstacksxe2x80x9d including a number of single fuel cells joined to form a cell stack to increase the voltage potential to meet specific electrical power requirements. The feasibility of using DMFC systems as alternative power sources for portable electronics applications will depend upon the reduction of the size of the overall system to meet demanding form factors, while satisfying the necessary power requirements for electrical power applications.
In addition, DMFC systems useful for consumer electronics applications will require development and design engineering that will enable methanol fuel cells to self-regulate and passively generate electrical power under relevant operating conditions, including ambient air temperature and humidity with a minimum of active humidity or temperature regulation. Such operating conditions may further require the reduction or elimination of auxiliary equipment and external moving parts typically associated with present DMFC systems, such as external fins for heat dissipation, fans for cooling and external flow pumps for supplying pressurized gas reactants and water for sufficient membrane humidification. In addition, the volume of peripheral mechanisms or systems, such as pumps and reservoirs used to store and supply methanol fuel and gas separators used to remove gases from liquid fuel cell effluents, will need to be reduced or eliminated in DMFC systems for portable power and consumer electronics applications.
At present, prior art DMFC systems typically operate in two basic configurations, a flow-through configuration and a recirculation configuration, as disclosed, for example in U.S. Pat. Nos. 5,992,008, 5,945,231, 5,795,496, 5,773,162, 5,599,638, 5,573,866 and 4,420,544. The flow-through configuration directly feeds methanol as a vapor or a stream of either neat methanol or an aqueous solution of methanol and water into the anode electrode of the fuel cell. Anodic oxidation by-products, specifically carbon dioxide, as well as fuel impurities and unreacted methanol fuel solution are removed from the fuel cell to the ambient environment. The flow-through configuration has the disadvantages of wasting unused fuel, and making it difficult to manage effluent by-products. In addition, the flow-through configuration presents problems with respect to handling the anode effluent discharged from the fuel cell. Peripheral mechanisms or systems are required with the flow-through configuration of DMFC systems to remove and dispose of the anode effluent discharged from the fuel cell. Such mechanism or systems would render flow-through DMFC systems impractical for use in portable electronics applications.
The recirculation configuration of DMFC systems, however, has the advantages of recirculating the anode effluent back into the anode electrode, which conserves unused methanol fuel and contains the anode effluent generated by the electrochemical oxidation/reduction processes.
Prior art DMFC systems with recirculation configurations address the problems of handling anode effluent, conserving unused methanol fuel and providing a means of managing by-products of the reaction. Such features are highly advantageous for use of DMFC systems in portable power supplies and portable consumer electronics. However, recirculation configurations of prior art DMFC systems must incorporate auxiliary or external peripheral equipment in the recirculation loops that occupy volume and add complexity to DMFC systems due to their use of electrical power, thus limiting the net power output of the DMFC system.
In a DMFC, it is necessary to provide sufficient quantities of fuel (a mixture of water and methanol) to the catalyzed anode face of the PCM, and oxygen to the catalyzed cathode face of the PCM. Failure to allow sufficient quantities of the reactants to be introduced to the PCM results in the cessation of the reactions that generate electricity in a fuel cell.
During operation, the cathode diffusion layer may become saturated by water that is generated on the cathode face of the PCM. The saturation of the diffusion layer prevents oxygen from reaching the cathode face of the PCM, and causes the fuel cell""s performance to be compromised, or be halted altogether. In addition, particulate airborne contaminants may clog the openings in the structure of the cathode diffusion layer, thus further limiting the performance of the DMFC. U.S. Pat. No. 6,326,097 to Hockaday discloses the use of a disposable diffusion mat that is integrated into the external housing of the fuel cell system and is used to manage temperature and humidity, and also provides some filtering characteristics as well.
Moreover, it is desirable to supply methanol to the catalyzed PCM in an aqueous solution. By doing so, methanol crossover (a process whereby methanol passes through the PCM without contributing to the generation of electricity) is reduced and the overall utility of the cell is enhanced. It is desirable to carry a more concentrated mixture of fuel, rather than a dilute mixture, for the purpose of minimizing the volume of the fuel cartridge or internal reservoir, and therefore the system. However, to ensure that the DMFC will continue generating electricity, sufficient oxygen must be supplied to the cathode, and under certain operating conditions, it may be necessary to provide a method to facilitate the removal of cathodically generated water and any other fluids that may be present on the cathode face of the PCM, or within the cathode diffusion layer of the fuel cell, including but not limited to liquid water that may be present.
Previously disclosed methods of water removal from the cathode diffusion layer of the fuel cell rely on wicking, drying, or absorption to transport or remove cathodically generated water, or are gravity dependent. The effectiveness of these methods varies with the ambient air temperature and humidity, or the orientation of the device and are difficult to control or vary predictably. As such they are less than ideal for use in portable electronics which will be exposed to a wide variety of conditions during use, and which require that they remain operational during periods of rapid change of such conditions.
There remains a need, therefore, for a direct oxidation fuel cell system that optimizes oxygen being provided to the cathode and prevents excess water accumulation on the cathode face of the PCM and the cathode diffusion layer of the fuel cell. Therefore, it would be desirable to provide a method and apparatus for preventing excess water from saturating the cathode diffusion layer and thereby preventing oxygen from reaching the cathode catalyst layer, and preferably recirculating the excess water to adjust the fuel concentration within the fuel cell system, enabling the system to carry a more concentrated fuel source.
One aspect of the present invention relates to a system including: a direct oxidation fuel cell, which includes a housing surrounding an MEA, a current collector disposed on the outside of the MEA to collect and conduct electrical current to a load, and a gas-permeable liquid-impermeable membrane disposed on a cathode-side outer surface of the current collector, wherein the MEA includes an anode aspect, a cathode aspect, and a PCM disposed between the anode aspect and the cathode aspect; a source of fuel in communication with the anode aspect; a source of oxygen in communication with the cathode aspect, so as to produce electricity-generating reactions, including anodic disassociation of a fuel and water mixture to produce carbon dioxide, protons and electrons and a cathodic combination of protons, electrons and oxygen to produce water; and a pump in fluid communication with an area between the PCM and the gas-permeable liquid-impermeable membrane, connected to remove excess water produced at the cathode aspect.
Another aspect of the present invention relates to a system including a direct oxidation fuel cell, which includes a housing surrounding an MEA, including an anode diffusion layer, a cathode diffusion layer, and a PCM disposed between the anode diffusion layer and the cathode diffusion layer, a catalyst layer is preferably disposed on one or both faces of the PCM in intimate contact with the PCM and the diffusion layer, a current collector disposed on the outside of the MEA to collect and conduct electrical current to a load, and a gas-permeable liquid-impermeable membrane disposed on an outer cathode-side surface of the current collector; a source of fuel in communication with the anode aspect of the MEA; a source of oxygen in communication with the cathode aspect of the MEA, so as to produce electricity-generating reactions, including anodic disassociation of a fuel and water mixture to produce carbon dioxide, protons and electrons and a cathodic combination of protons, electrons and oxygen to produce water; and a pump in fluid communication with an area between the PCM and the gas-permeable liquid-impermeable membrane, connected to remove excess liquid water produced at the cathode.
Another aspect of the present invention is a method for managing water in a direct oxidation fuel cell, including: providing a direct oxidation fuel cell, including: a housing surrounding an MEA, wherein the MEA includes an anode aspect, a cathode aspect and a PCM disposed between the anode aspect and the cathode aspect, a current collector disposed on the outside of the MEA to collect and conduct electrical current to a load, and a gas-permeable liquid-impermeable membrane disposed on a cathode-side outer surface of the current collector; providing fuel to the anode aspect of the fuel cell; providing oxygen to the cathode aspect of the fuel cell; and removing excess water accumulation from an area between the PCM and the gas-permeable liquid-impermeable membrane.
Another aspect of the present invention relates to a method for managing water in a direct oxidation fuel cell, including: providing a direct oxidation fuel cell, including a housing surrounding an MEA, wherein the MEA includes an anode diffusion layer, a cathode diffusion layer, and a PCM disposed between the anode diffusion layer and the cathode diffusion layer, a catalyst layer is preferably disposed on one or both faces of the PCM in intimate contact with the PCM and the diffusion layer, a current collector disposed on the outside of the MEA to collect and conduct electrical current to a load, and a gas-permeable liquid-impermeable membrane disposed on an outer cathode-side surface of the current collector; providing fuel to the anode diffusion layer of the MEA; providing oxygen to the cathode diffusion layer of the MEA; and removing excess water accumulation from an area between the PCM and the gas-permeable liquid-impermeable membrane.
Another aspect of the present invention relates to a method including: providing a direct oxidation fuel cell, including a housing surrounding an MEA; wherein the MEA includes an anode aspect, a cathode aspect and a PCM disposed between the anode aspect and the cathode aspect, a current collector disposed on the outside of the MEA to collect and conduct electrical current to a load, and a gas-permeable liquid-impermeable membrane disposed on a cathode-side outer surface of the MEA; providing fuel to the anode aspect of the current collector; providing oxygen to the cathode aspect of the PCM; and drawing air to the surface of, into or through the cathode diffusion layer, preferably, via the creation of a pressure differential.
Additional features of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed.